Sarcoma is a type of cancer in dogs that develops from connective tissues like muscle, fat, blood vessels, or bone. Unlike carcinomas, which arise from the cells lining organs and skin surfaces, sarcomas grow from the structural tissues that hold the body together. They’re among the most common cancers in dogs, and they range from slow-growing lumps under the skin to aggressive tumors that spread quickly to other organs.
How Sarcomas Differ From Other Cancers
All sarcomas originate in what pathologists call mesenchymal tissue. This is the broad category of connective tissue that includes bone, cartilage, fat, muscle, and blood vessels. Because these tissues exist throughout the body, sarcomas can appear almost anywhere.
The two major categories you’ll hear about are soft tissue sarcomas and bone sarcomas. Soft tissue sarcomas are a group of tumors that look and behave similarly despite coming from different cell types. They typically show up as lumps under the skin on the trunk, legs, or head. Osteosarcoma, the most common bone sarcoma, usually strikes the long bones of the legs and tends to be far more aggressive.
Common Types in Dogs
Several specific sarcoma types appear frequently in veterinary practice:
- Soft tissue sarcomas include fibrosarcoma (from fibrous tissue), peripheral nerve sheath tumors (from nerve coverings), and perivascular wall tumors (from blood vessel walls). These are often grouped together because they share similar behavior and treatment approaches.
- Hemangiosarcoma develops from the cells lining blood vessels. It commonly affects the spleen, heart, and liver, making it one of the most dangerous sarcomas because it can rupture and cause internal bleeding with little warning.
- Osteosarcoma is a bone cancer that primarily affects large and giant breed dogs. It’s painful and highly likely to spread to the lungs.
What a Sarcoma Looks and Feels Like
Soft tissue sarcomas usually appear as a lump or mass under the skin. The texture varies. Some feel firm and seem anchored to the tissue beneath them, while others are softer and move around when you push on them. Growth rate differs too. Some masses grow so slowly that owners assume they’re harmless for months, while others seem to appear and enlarge within weeks.
Other signs depend on where the tumor sits. A mass on a leg can interfere with walking. One pressing against a nerve may cause pain. Some tumors eventually break through the skin, creating open wounds that are prone to infection and discharge. Many dogs with soft tissue sarcomas show no signs of illness beyond the lump itself, at least early on.
Osteosarcoma is different. It typically causes lameness and swelling at the affected bone, and the pain can be significant. Because it weakens the bone from within, some dogs are first diagnosed after a fracture that happens during normal activity.
How Sarcomas Are Diagnosed
If your vet finds a suspicious lump, the first step is usually a fine-needle aspiration. A small needle is inserted into the mass to collect cells, which are examined under a microscope. This quick, minimally invasive test can often determine whether the mass is benign (like a fatty lipoma) or something more concerning. It’s good at categorizing a mass as coming from connective tissue, but it rarely provides a specific sarcoma diagnosis or grade on its own.
For a definitive answer, a tissue biopsy is needed. A larger sample of the tumor is removed and sent to a pathologist, who can identify the exact tumor type, how aggressive the cells look, how quickly they’re dividing, and whether they’re invading surrounding tissue. This information directly shapes treatment decisions.
Once a sarcoma is confirmed, staging determines whether it has spread. This typically involves chest X-rays to check the lungs (the most common site for distant spread), abdominal ultrasound, and sampling of nearby lymph nodes. Normal-sized lymph nodes can still contain cancer cells, so physical exam alone isn’t reliable for ruling out spread. CT or MRI scans may be recommended to map the tumor’s exact boundaries before surgery, particularly for tumors near complex anatomy.
Tumor Grading and What It Means
Soft tissue sarcomas are assigned a grade from I to III based on three factors: how closely the tumor cells resemble normal tissue, how fast the cells are dividing, and how much of the tumor has died internally (necrosis). Each factor gets a score, and the total determines the grade.
Grade I (low grade) tumors are the least aggressive. They grow slowly, and their cells still look relatively normal. Grade III (high grade) tumors are at the other end of the spectrum, with rapidly dividing, abnormal-looking cells. The grade matters most for predicting spread. In one study of internal sarcomas, the rate of spread to other organs was 20% for grade I tumors, 27% for grade II, and 60% for grade III. Grade also influences whether your vet recommends chemotherapy after surgery.
Surgery as the Primary Treatment
Wide surgical removal is the cornerstone of sarcoma treatment. The goal is to remove the entire tumor along with a margin of healthy tissue around it to reduce the chance of regrowth. Recommended margins in veterinary oncology range from a few millimeters to 5 centimeters of surrounding tissue, depending on tumor type, grade, and location, plus at least one layer of intact tissue (called a fascial plane) beneath the tumor.
Achieving clean margins can be straightforward for a small, well-placed tumor on the trunk, but it becomes challenging when the mass is on a leg, near joints, or close to the face. When clean margins aren’t achievable, additional surgery or radiation therapy is typically recommended to address any remaining cancer cells.
For osteosarcoma, surgery usually means amputation of the affected limb. Most dogs adapt remarkably well to three legs, but the surgery alone doesn’t address the disease’s tendency to spread. With amputation alone, the median survival time is about four months. When chemotherapy is added after surgery, that extends to roughly nine months.
Radiation and Chemotherapy
Radiation therapy is most commonly used when a soft tissue sarcoma couldn’t be completely removed or when surgical margins were uncomfortably narrow. It targets the surgical site to destroy remaining cancer cells and significantly reduces the risk of the tumor growing back in the same spot.
Chemotherapy plays a more limited role for most soft tissue sarcomas compared to other cancers. For high-grade (grade III) tumors, it may be recommended after surgery because of the elevated risk of spread, though evidence that it extends survival in dogs is less clear than it is in human medicine. A low-dose daily chemotherapy approach, sometimes combined with an anti-inflammatory medication, has shown promise in improving the time before tumors recur after incomplete surgical removal.
For osteosarcoma, chemotherapy after amputation is standard because the cancer has almost always sent microscopic cells to the lungs by the time of diagnosis, even when imaging looks clean.
Recovery After Surgery
Most dogs need restricted activity for one to two weeks following tumor removal surgery, until sutures are removed. During this period, running, jumping, swimming, and bathing should all be avoided. The surgical site needs to stay clean and dry to prevent infection.
Watch the incision daily. Contact your veterinarian if you notice continuous blood drainage, excessive swelling or redness, an unpleasant odor, or any discharge from the wound. Blood seepage that continues beyond 24 hours also warrants a call.
After recovery, your vet will likely schedule regular follow-up exams every two to three months. These visits typically include feeling the surgical site for any new lumps and periodic chest X-rays to screen for spread. Soft tissue sarcomas that are going to recur locally tend to do so within the first year, so this is the period of closest monitoring. Knowing what the original tumor felt like helps you check at home between appointments. Any new firm mass at or near the old surgical site deserves prompt evaluation.
Prognosis Depends on the Type and Grade
Outcomes vary enormously depending on the specific sarcoma. Low-grade soft tissue sarcomas that are completely removed with wide margins have an excellent prognosis. Many of these dogs live normal lifespans and never see the tumor return. When margins are incomplete, local recurrence is the main concern, though repeated surgery or radiation can still achieve long-term control in many cases.
High-grade soft tissue sarcomas carry a meaningful risk of spreading to the lungs or other organs, which worsens the outlook considerably. Hemangiosarcoma, regardless of treatment, tends to carry a poor prognosis because it’s often widespread by the time it’s discovered. Osteosarcoma, even with aggressive treatment combining amputation and chemotherapy, has a median survival of about nine months, though some dogs live well beyond that.
The single most useful thing you can do for your dog is catch lumps early. A mass the size of a grape is far easier to remove with clean margins than one that’s been growing for months. Any new lump that persists for more than two to three weeks, grows noticeably, or feels firmly attached to deeper tissue is worth having evaluated promptly.